The invention relates to a linear light source, and more particularly to such a linear light source for use in a film scanner.
Light integrating chambers for film scanning are known in the prior art. A conventional cylindrical integrating cavity used in a film scanner is shown in FIG. 1. As currently used as linear light sources in many film scanners, light from a lamp 12 and a lens 14 is directed into a hole 16 in a cylinder wall of an integrating cylinder 10. The light reflects many times against the white interior chamber wall of the cylinder 10 to randomize its distribution. This randomization is intended to produce uniform, diffuse light within the cylinder 10, which then exits through a long, narrow exit slit 18 to produce a stripe of uniform, diffuse illumination applied against a surface of an original material, such as a film 20. The light passing through the film 20 is modulated in intensity by the density varying image on the film and focused by a lens system 21 on a sensor array 24.
It is desirable to produce a highly intense, uniform and lambertian line source of illumination for use in a film scanner application. A further requirement for an illumination system is that the efficiency be as high as possible while still providing the desired uniformity and diffusion. This can be achieved by designing the cavity to have the highest possible reflectivity of the walls and the minimum possible area for the introduction of the light. Bulk diffusers such as Spectralon currently have the lowest loss of available reflective diffuse materials. Furthermore, an integrating cylinder apparatus that allows a small interior surface area and entrance hole area while maintaining a desired intensity profile by using a light pipe to couple light into the cavity has been disclosed in U.S. Pat. No. 5,274,228 (M. Kaplan). As shown in FIG. 2, this type of illuminator allows light into a cylindrical cavity 34 via a glass rod 22 which is positioned along the longitudinal direction of the cylindrical cavity 34. Light 28 enters the rod 22 at one end 26 and then propagates inside the rod via total internal reflection (TIR) towards the opposite end 29 of the rod. The rod is given a surface treatment 30 that disrupts the TIR and diffusely leaks light 32 into the cavity interior where it is diffused further and eventually finds its way out the exit slit 18. The amount of light released into the cavity interior at any particular spot then is roughly proportional to the degree or width of the surface treatment 30 but is also influenced by the characteristics of the input light angular component.
The method of the ""228 patent performs well when the input light contains a wide range of input angles as the higher angled light more easily exits the rod at the surface treatment 30 than does shallow angled light. This effect implies that after light has traveled down the length of the rod much of its higher angled light exits the rod leaving a more collimated beam at the end of the rod. Further, it implies that there is still a substantial amount of light at the end of the rod which if allowed to flood the end of the cavity would produce a bright spot at the end of the cavity. It is desirable then to diffusely reflect this lower angled light using a reflecting mirror 36 at the end of the rod back into the rod for another chance of exiting the rod in more useful locations along the rod. (Alternatively, the mirror 36 may be replaced by a white, diffusely reflecting surface spaced at least a few wavelengths of light away from the end of the rod.) The act of diffusing the light at the end of the rod redistributes the angular components so as to produce more higher angled light which has a better chance at exiting the rod at locations along its surface treatment.
Furthermore, it is desirable to be able to attenuate the intensity of the output light from the illuminator without undue adverse effects on the spatial profile exiting the illuminator slit. The range of desired attenuation is typically somewhere around five to six F-stops. This is accomplished by attenuating the light before it enters the cavity via its glass rod component. According to this method of attenuation, a pair of movable V-shaped blades 34 (see FIG. 2) are placed in the light path so as to provide a means of varying the amount of light passing through the aperture. In so doing, the collection of input light path angles becomes truncated as the aperture is closed down. This causes a change in the source light angular component entering the integrator cavity. Changing the aperture opening varies the amount of light likely to reach the end of the rod and thus also effects the spatial profile as a function of the aperture opening. This is an undesirable effect. In most scanners that use the known types of illuminators, however, the problem is unaddressed because they do not require attenuation of the light source.
As mentioned above, the concept of diffusely reflecting light back into the rod is generally disclosed in the ""228 patent; however, the disclosed concept involves drawbacks that are neither identified nor described. For example, by not holding/capturing the end of the rod or by allowing a space between the end of the rod and the far end of the cavity interior, a bright spot is generated at the far end of the cavity due to excessive flooding with light. Conversely, attempting to capture the end of the rod in a bore slightly bigger than the rod provides no support for the rod and still allows some flooding to occur. Attempting to capture the end of the rod inside a bore of the same size as the rod where the rod snugly fits into the bore fails when the tolerance of the diameter of the rod exceeds the diameter of the bore; a slightly oversized rod will not fit into the bore. Furthermore, when a high power light source is used, a considerable amount of energy reaches the end of the rod and may damage the diffusely reflective material/coating at the surface of the cavity if insufficient cooling is provided such as forced air injected into the cavity. If the end of the rod is captured in a snug fitting bore there is little chance of circulating cooling air to get around the end of the rod.
Currently, illuminators that use glass rods in cavities use various techniques to hold the rod inside the cavity. A) As shown in FIG. 2, some designs use no interior end holding feature; the rod is held at the input end only. This means that the (cavity interior) end of the rod is free to wiggle or bottom out via gravity to the bottom of the cavity. This method has been used where the cavity is only slightly larger (in diameter) than the rod and when the design does not need to operate over a wide range of attenuation. Often, no special attention is given to recoupling the light at the end of the rod other than its close proximity to the end wall (of diffusely reflective material). B) At least one other design uses a xe2x80x9cVxe2x80x9d shaped end feature to help center the rod at the end of the cavity and provide a minimal contact to the rod material. Again, this design does not require a wide range of attenuation, as does the design mentioned above. Also, when using a xe2x80x9cVxe2x80x9d shaped feature, there is no guarantee that the rod, due to length tolerances, will in fact engage the xe2x80x9cVxe2x80x9d shaped edges. Usually the input end of the rod is constrained to a particular dimension/position and the far end of the rod (inside the cavity) is allowed to vary in position due to the rod length tolerances. C) Some designs have attempted to use a bore in the end face wall of the cavity to allow the rod to fit inside the bore. The problem with this design is that if the bore is selected to have a diameter the same as the rod diameter then some rods will not fit into the bore if they are slightly over-sized. Conversely, if the rod diameter is less than the diameter of the bore, then the rod is in effect not held or constrained.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a light integrator comprising: an elongated cylindrical light integrating cavity having a longitudinal cylindrical chamber wall with a diffusely reflecting interior surface, said chamber wall having a longitudinally extending output slit for emitting light from the cavity; an elongated glass rod extending into the cavity, said glass rod having an input port at one end thereof for introducing the beam of light and a treatment along its length for emitting light entering its port into the cavity; and an end wall including a supporting feature for supporting an end of the elongated glass rod opposite its input port so that the glass rod extends along a length of the integrating cavity in relation to the chamber wall thereof to direct light emitted therefrom toward the diffusely reflecting interior surface, whereby the supporting feature comprises a bore that is recessed into the end wall with a linear dimension leaving an enclosed open space between the end of the glass rod and the end wall of the cavity for diffusely reflecting light reaching the end of the rod back into the rod for another opportunity to exit the rod through the treatment.
In an alternative aspect of the invention, the supporting feature comprises a bore that is recessed into the end wall and includes a flexible wall structure extending outward from the end cap and providing a semi-rigid support for the end of the rod. In yet another alternative aspect of the invention, the supporting feature comprises a multi-sided bore that is recessed into the end wall and provides open spaces at corners thereof when the end of the glass rod is received into the bore.
The advantage of the invention is that it provides robust means for holding the end of a rod inside the cavity, while at the same time improving the dynamic shading performance while allowing various diameter tolerance rods to fit the holder structure.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.